1. Field of the Invention
In general, the present invention relates to the production of zeolites.
2. Description of the Prior Art
Certain naturally occurring hydrated metal aluminum silicates are called zeolites. The synthetic adsorbents of the invention have compositions similar to some of the natural zeolites. The most common of these zeolites are sodium zeolites. Zeolites are useful as detergent builders, cracking catalysts and molecular sieves.
Zeolites consist basically of a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra. The tetrahedra are cross-atoms to the total of the aluminum and silicon atoms is equal to two or O/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion of the crystal of a cation, for example, a sodium ion. This balance may be expressed by the formula Al/Na=1. The spaces between the tetrahedra are occupied by water molecules prior to dehydration.
Zeolites "X" and "Y" may be distinguished from other zeolites and silicates on the basis of their X-ray powder diffraction patterns and certain physical characteristics. The X-ray patterns for several of these zeolites are described below. The composition and density are among the characteristics which have been found to be important in identifying these zeolites.
The basic formula for all crystalline sodium zeolites may be represented as follows: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :xSio.sub.2 :yH.sub.2 O.
In general, a particular crystalline zeolite will have values for x and y that fall in a definite range. The value x for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. Minor variations in the relative numbers of these atoms do not significantly alter the crystal structure or physical properties of the zeolite. For zeolite X, an average value for x is about 2.5 with the x value falling within the range 2.5.+-.0.5.
The value of y is not necessarily an invariant for all samples of zeolites. This is true because various exchangeable ions are of different size, and, since there is no major change in the crystal lattice dimensions upon ion exchange, the space available in the pores of the zeolite to accommodate water molecules varies.
The average value for y determined for zeolite X is 6.2.
The formula for zeolite X may be written as follows: EQU 0.9.+-.0.2N.sub.2 O:Al.sub.2 O.sub.3 :2.5.+-.0.5SiO.sub.2 :yH.sub.2 O;
The formula for zeolite Y may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :4.5.+-.1.5SiO.sub.2 :yH.sub.2 O; and,
"y" may be any value up to 8 for zeolite X and any value up to 9 for zeolite Y.
The pores of zeolites normally contain water.
The above formulas represent the chemical analysis of zeolites X and Y. When other materials as well as water are in the pores, chemical analysis will show a lower value of y and the presence of other adsorbates. The presence in the crystal lattice of materials volatile at temperatures below about 600.degree. C. does not significantly alter the usefulness of the zeolites as an adsorbent since the pores are usually freed of such volatile materials during activation.
Among the ways of identifying zeolites and distinguishing them from other zeolites and other crystalline substances, the X-ray powder diffraction pattern has been found to be a useful tool. In obtaining X-ray powder diffraction patterns, standard techniques are employed. The radiation is the Ka doublet of copper, and a Geiger counter spectrometer with a strip chart pen recorder is used. The peak heights, I, and the positions as a function of 2.theta. where .theta. is the Bragg angle, are read from the spectrometer chart. From these, the relative intensities, 100I/I.sub.0, where I.sub.0 is the intensity of the strongest line or peak, and d the interplanar spacing in A corresponding to the recorded lines are calculated.
X-ray powder diffraction data for sodium zeolite X are given in Table A. 100I/I.sub.0 and the d values in angstroms (A) for the observed lines for zeolite X are also given. The X-ray patterns indicate a cubic unit cell of dimensions between 24.5 A and 25.5 A. In a separate column are listed the sum of the squares of the Miller indices (h.sup.2 +k.sup.2 +l.sup.2) for a cubic unit cell that corresponds to the observed lines in the X-ray diffraction patterns. The a.sub.0 value for zeolite X is 24.99 A, where a.sub.0 is the unit cell edge.
TABLE A ______________________________________ X-RAY DIFFRACTION PATTERN FOR SYNTHETIC FAUJASITE (ZEOLITE X) h.sup.2 + k.sup.2 + l.sup.2 ##STR1## d (A) ______________________________________ 3 100 14.47 8 18 8.85 11 12 7.54 19 18 5.73 27 5 4.81 32 9 4.42 35 1 4.23 40 4 3.946 43 21 3.808 44 3 3.765 48 1 3.609 51 1 3.500 56 18 3.338 59 1 3.253 67 4 3.051 72 9 2.944 75 19 2.885 80 8 2.794 83 2 2.743 88 8 2.663 91 3 2.620 96 1 2.550 108 5 2.404 123 1 2.254 128 3 2.209 131 3 2.182 136 2 2.141 139 2 2.120 144 1 2.038 164 1 1.952 168 1 1.928 184 1 1.842 195 1 1.789 200 2 1.767 211 3 1.721 243 3 1.603 ______________________________________
The more significant d values for zeolite X are given in Table B.
TABLE B ______________________________________ MOST SIGNIFICANT d VALUES FOR ZEOLITE X d Value of Reflection in A ______________________________________ 14.45 .+-. 0.2 8.85 .+-. 0.1 7.55 .+-. 0.1 5.75 .+-. 0.1 4.42 .+-. 0.05 3.80 .+-. 0.05 3.33 .+-. 0.05 2.88 .+-. 0.05 2.79 .+-. 0.05 2.66 .+-. 0.05 ______________________________________
Zeolite Y has a characteristic X-ray powder diffraction pattern which may be employed to identify zeolite Y. The X-ray powder diffraction data are shown in Table C. The values for the interplanar spacing d are expressed in angstrom units.
Occasionally, additional lines not belonging to the pattern for the zeolite appear in a pattern along with the X-ray lines characteristic of that zeolite. This is an indication that one or more additional crystalline materials are mixed with the zeolite in the sample being tested. Frequently, these additional materials can be identified as initial reactants in the synthesis of the zeolite, or as other crystalline substances. When the zeolite is heat treated at temperatures of between 100.degree. C. and 600.degree. C. in the presence of water vapor or other gases or vapors, the relative intensities of the lines in the X-ray pattern may be appreciably changed from those existing in the unactivated zeolite patterns. Small changes in line positions may also occur under these conditions. These changes in no way hinder the identification of these X-ray patterns as belonging to the zeolite.
TABLE C ______________________________________ X-RAY DIFFRACTION PATTERN FOR SYNTHETIC FAUJASITE (ZEOLITE Y) Relative h.sup.2 + k.sup.2 + l.sup.2 d (A) Intensity ______________________________________ 3 14.29 100 8 8.75 9 11 7.46 24 19 5.68 44 27 4.76 23 32 4.38 35 40 3.91 12 43 3.775 47 48 3.573 4 51 3.466 9 56 3.308 37 59 3.222 8 67 3.024 16 72 2.917 21 75 2.858 48 80 2.767 20 83 2.717 7 88 2.638 19 91 2.595 11 108 2.381 6 123 2.232 2 128 2.188 4 131 2.162 3 139 2.099 5 144 2.062 3 164 1.933 2 168 1.910 3 179 1.850 2 187 1.810 2 192 1.786 1 195 1.772 2 200 1.750 4 211 1.704 5 ______________________________________
The particular X-ray technique and/or apparatus employed, the humidity, the temperature, the orientation of the powder crystals and other variables, all of which are well known and understood to those skilled in the art of X-ray crystallography or diffraction can cause some variations in the intensities and positions of the lines. These changes, even in those few instances where they become large, pose no problem to the skilled X-ray crystallographer in establishing identities. Thus, the X-ray data give herein to identify the lattice for a zeolite, are not to exclude those materials, which, due to some variable mentioned or otherwise known to those skilled in the art, fail to show all of the lines, or show a few extra ones that are permissible in the cubic system of that zeolite, or show a slight shift in position of the lines, so as to give a slightly larger or smaller lattice parameter.
A simple test described in "American Mineralogist," Vol. 28, Page 545, 1943, permits a quick check of the silicon to aluminum ratio of the zeolite. According to the description of the test, zeolite minerals with a three-dimensional network that contains aluminum and silicon atoms in an atomic ratio of Al/Si=2/3 =0.67, or greater, produce a gel when treated with hydrochloric acid. Zeolites having smaller aluminum to silicon ratios disintegrate in the presence of hydrochloric acid and precipitate silica. These tests were developed with natural zeolites and may vary slightly when applied to synthetic types.
U.S. Pat. No. 2,882,244 describes a process for making zeolite X comprising preparing a sodium-aluminum-silicate water mixture having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of from 3:1 to 5:1, an Na.sub.2 O/SiO.sub.2 mole ratio from 1.2:1 to 1.5:1, and an H.sub.2 O/Na.sub.2 O mole ratio of from 35:1 to 60:1, maintaining the mixture at a temperature of from 20.degree. C. to 120.degree. C. until zeolite X is formed, and separating the zeolite X from the mother liquor.
In U.S. Pat. No. 3,119,659, a kaolin clay and sodium hydroxide are formed into a compact body, dried, reacted in an aqueous mixture at a temperature of from 20.degree. C. to 175.degree. C. until a zeolite is formed. Zeolite X is formed in a reaction mixture having an Na.sub.2 O/SiO.sub.2 molar ratio of 1.5:1, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 5:1, and an H.sub.2 O/Na.sub.2 O molar ratio of 30:1 to 60:1. Zeolite Y is formed in a reaction mixture having an Na.sub.2 O/SiO.sub.2 molar ratio of 0.5:1, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 7:1, and an H.sub.2 O/Na.sub.2 O molar ratio of 20:1 to 40:1.
U.S. Pat. No. 3,920,789 discloses a process for making zeolite Y using elevated temperatures and pressures for the crystallization stage followed by very rapid cooling of the reaction mass.
In U.S. Pat. No. 3,130,007, zeolite Y is formed by preparing an aqueous sodium alumino silicate mixture having a certain composition, maintaining the mixture at a temperature of 20.degree. C. to 125.degree. C. until zeolite Y is formed, and separating the zeolite Y from the mother liquor. Table D shows reaction mixture compositions that produce zeolite Y.
TABLE D ______________________________________ U.S. Pat. No. 3,130,007 REACTION MIXTURE COMPOSITIONS FOR ZEOLITE Y Na.sub.2 O/SiO.sub.2 SiO.sub.2 /Al.sub.2 O.sub.3 H.sub.2 O/Na.sub.2 O ______________________________________ 0.20-0.40 10-40 25-60 0.41-0.60 10-30 20-60 0.61-0.80 7-30 20-60 0.6-1.0 8-30 12-90 1.5-1.7 10-30 20-90 1.9-2.1 10 40-90 ______________________________________
U.S. Pat. No. 3,130,007 indicates in Column 2, lines 35-42, the necessity of using an active silica source by specifying that aqueous colloidal silica sols or reactive amorphous solid silicas are preferred.
In U.S. Pat. No. 4,016,246, zeolite Y is formed by preparing an aqueous alumino silicate reaction mixture by mixing an alumina component and an Na.sub.2 O component with an active hydrate sodium metasilicate to form a certain reaction mixture, then heating the mixture at a temperature of 20.degree. C. to 120.degree. C. until zeolite Y is formed. Table E shows reaction mixture compositions that produce zeolite Y.
TABLE E ______________________________________ U.S. Pat. No. 4,016,246 REACTION MIXTURE COMPOSITIONS FOR ZEOLITE Y Na.sub.2 O/SiO.sub.2 SiO.sub.2 /Al.sub.2 O.sub.3 H.sub.2 O/Na.sub.2 O ______________________________________ 0.28-&lt;0.30 8-10 20-70 0.30-&lt;0.31 8-12 20-70 0.31-&lt;0.32 8-14 20-70 0.32-&lt;0.34 8-16 12-90 0.34-&lt;0.40 7-40 12-120 0.4-&lt;0.7 5-50 12-120 0.7-&lt;1.0 31-50 12-120 ______________________________________
U.S. Pat. No. 4,016,246 also discusses the significance of using an activated source of sodium silicate. In such patent an active hydrated sodium metasilicate is prepared by carefully hydrating sodium metasilicate under specified conditions.
U.S. Pat. No. 4,166,099 discloses a process for preparing crystalline aluminosilicate zeolites, particularly synthetic faujasites such as zeolite type X and zeolite type Y utilizing especially prepared nucleation centers or seeds. Such seed preparation is lengthy and involved.
U.S. Pat. No. 4,164,551 discloses a process for making zeolite Y also utilizing specially prepared nucleation centers.
From the prior art, one would assume that zeolite X cannot be made from reaction mixtures having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio greater than 5:1 and that zeolite Y cannot be made from an unreactive source of silica.